
* unlink & open with the 'w' flag should be used with extrem care. 

* anything under the cache directory (.axvpixs)  can be unlinked
or overwritten.

* threads should be reused

* when rendering previews from the cache, the dimensions
of the original picture (not the cache) are lost.
There's 3 alternative to fix this bug:
	1) put them in the cached picture either
	as a jpeg comment or by creating a jpeg
	based file format
	2) put them in a separated file (say '.index'
	in the cache directory
	3) reread them from the original picture
each of these solutions have advantages & disavantages

* there should be 2 pixmaps for pictures with an
alpha channel (1 combined with the 'normal' background
and another combined with the 'selected' background)

* some memory leaks appear when you close the application
while it's rendering some pictures. The object's
'finalize' methods doesn't seem to be invoked.
This is awfull because some X pixmaps won't be 
free and will stay in the XServer memory (?)

* Warning: It is assumed that 'guint' is an atomic data type

